Clothes-washing and spin-drying machines

ABSTRACT

A clothes-washing and spin-drying machine having a drum rotatable about a nonvertical axis at a relatively slow speed for washing and at a much higher speed for spin drying, including control means for accelerating the driving motor, and hence the drum, relatively slowly through a speed at which the centrifugal force on the clothes within the drum equals the force due to gravity, to produce a uniform distribution of the clothes within the drum, and subsequently more rapidly towards the spin-drying speed.

[45] Feb. 8, 1972 United States Patent Eastall ...68/24 XMellinger...............................68/24 X [54] CLOTHES-WASHING ANDSPIN- 3,055,203

DRYING MACHINES 3,293,889 12/1966 Cobb et al.. [72] Inventor: MartinHarold Eastall, Peterborough, En- 3,346,115 10/1967 gland [73] Assignee:British Domestics Appliances Limited,

Primary Examiner-Marvin A. Champion Assistant Examiner-Robert A. HaferPeterborough England Anamey-Larson, Taylor and Hinds June19,l969 V V.

Appl. No.: 834,686 g [22 Filed:

ABSTRACT A clothes-washing and spin-drying machine having a drum [30]Foreign Application Priority Data June 20, 1968 Great rotatable about anonvertical axis at a relatively slow speed for .29,412/68 washing andat a much higher speed for spin drying, including control means foraccelerating the driving motor. and hence .68/12 R, 68/24 the drum,relatively slowly through a speed at which the cen- 33/00 trifugal forceon the clothes within the drum equals the force .68/12, 24; 318/398, 416due to gravity, to produce a uniform distribution of the clothes withinthe drum, and subsequently more rapidly towards the References Citedspin-drying speed.

UNITED STATES PATENTS 3,003,090 10/1961 Neyhouse et 11 Claims, 2 DrawingFigures F Ii. .l... s? s i|.l!|i|.|.| |i|. Ii. 1 88 Tm w N w, a .74 I II.L e Q Fest Acceleration -Slow Acceleration 1 l l I l I l l m oeqm L226CLOTHES-WASHING AND SPIN-DRYING MACHINES This invention relates toclothes-washing and spin-drying machines and relates especially thoughnot exclusively to such machines of the type in which clothes can bewashed in a drum by means of a tumbling action effected by rotating thedrum about a horizontal or at least inclined axis at a relatively lowspeed in the presence of washing liquid and can subsequently be dried bya spinning operation achieved by rotating the drum about the same axisat a comparatively high speed.

A problem associated with such washing and spin-drying machines is theprevention of excessive out of balance forces on the drum duringhigh-speed spinning clue to clothes accumulating at one point in thedrum. With lower spinning speeds the problem can be overcome byincorporating a suspension for the outer container of such a design thatout of balance forces are absorbed. However, at higher spinning speeds,which are, in any event, desirable for the successful drying of theclothes, this solution is not practicable and arrangements must be madeto ensure that the clothes drum is not excessively out of balance duringsuch high-speed rotation. This may be achieved by making the clothesdrum such that it is selfbalancing-but that reduces the possible sizeand hence the capacity of the drum. Alternatively it can be arrangedthat whenever the clothes drum is required to rotate at a high spinningspeed this operation is preceded by rotation of the drum at a lowwashing speed in the same direction. This solution is not entirelysatisfactory, however, as the speed for washing the clothes must be lowenough to cause the clothes to tumble and not cling to the drum wall,i.e., the centrifugal force on the clothes must be less than thegravitational force, and as a result the clothes are not evenlydistributed around the drum wall, as is desired, if the drum is to be ina balanced condition for the subsequent high-speed spinning operation.

It is therefore desirable that the drum be rotated at a speedintermediate the tumbling speed and the spinning speed for a short timeprior to spinning the drum at high speed in order to ensure that theclothes distribute themselves equally around the internal periphery ofthe drum in a balanced manner. This has been achieved, for example, byproviding a drive motor for the drum capable of energization at threespeeds, these speeds being selected by a control arrangement for themachine such that, on termination of a tumbling action provided byrotation of the motor at its lowest speed the spin action provided bythe motor rotating at its highest speed is precededvby a short period ofrotation at an intermediate speed where the weight of the clothes isjust balanced by the centrifugal force on the clothes at this speed, thedesired result being that the clothes cease to tumble within the drumand contact the inner peripheral wall of the drum in a balanced manner.The drive motor is then accelerated to its highest (spinning) speed. Aproblem inherent in this arrangement is that there is no certainty thatthe clothes will distribute themselves in a balanced manner in the drumduring the period of rotation at the intermediate speed, as thecomparatively rapid acceleration of the drum from tumbling speed to thisintermediate speed tends to fling the clothes abruptly outwards to thewall of the drum in the pattern of distribution which they adopt for theprevious tumbling operation (which is very rarely uniform). As a resultof this the acceleration to spinning speed results in large out ofbalance forces on the drum.

The object of the present invention is to reduce the difficultyindicated in the preceding paragraph.

According to the present invention there is provided a clothes-washingand spin-drying machine or a spin-drying machine, having a clothes drumrotatable about a nonvertical axis, and an electric motor for rotatingthe drum both at a relatively slow speed and also at a much higher speedfor spin drying, wherein there is provided control means for the motorcapable of accelerating the motor relatively slowly through a criticalspeed at which the centrifugal force on the clothes within the drumequals the force-due to gravity, and sub-- uequently accelerating themotor more rapidly until said higher speed is attained.

Such an arrangement results in a more uniform distribution of theclothes within the drum before the latter attains full spin-dryingspeed, thereby reducing the possibility of excessive out of balanceforces being created, and as a result particularly high drum speeds arerendered possible.

In a preferred form of the invention design for a combined washing andspin-drying machine a common DC series electric motor is provided fordriving the clothes drum both for washing and for spin drying, and thecontrol means includes a motor speed control circuit having means forgenerating a series of electric pulses of which the conduction angle ormark to space ratio determines the power supply to, the motor, and saidcontrol means includes time-dependent means for varying said conductionangle or mark to space ratio in such a way as to produce said differentaccelerations.

Conveniently a controlled semiconductor rectifier device such as athyristor is provided in series with the electric motor with respect toinput terminals which are arranged to be connected to an alternatingcurrent supply, electric pulses being arranged to be applied to the gateof the device in appropriate half-cycles of the alternating currentsupply to fire the device and cause driving power to be supplied to themotor.

The control means in such a case is conveniently formed with solid statecontrol elements, in particular transistors, so connected and arrangedas to control the conduction angle or mark-to-space ratio of the pulsesapplied to the motor from the semiconductor rectifier device.

The control means preferably also includes feedback means which normallytend to maintain the motor rotating with a selected constant slow speed,said time-dependent means being arranged to override the feedback meanswhen a spin drying operation is initiated. i

It will be understood that the invention also includes within its scopecontrol units for controlling the speed of an electric motor driving theclothes drum of a clothes-washing and spindrying machine or aspin-drying machine in accordance with the invention. I

In order that the present invention may be clearly understood andreadily carried into effect it will now be described with reference tothe accompanying drawings in which:

FIG. 1 illustrates diagrammatically the program control circuit for acombined washing and spin-drying machine according to one example of theinvention, and

FIG. 2 is a graph representing the relationship of the rotational speedwith time for the clothes drum during the period of controlledacceleration from washing speed to spinning speed.

The machine itself is not illustrated in the figures but typically itcan comprise a drum mounted on a shaft in bearings so as to be rotatableon a horizontal axis within a liquid containing cylinder which issuspended by means of cooperating springs and dampers within an outercabinet. A DC electric drive motor of the series-type is secured to theliquid containing cylinder and is drivingly connected by means of a V ormulti-V belt-drive system to a pulley wheel secured to the drum shaftoutside the liquid cylinder. The system is such thatthe drum is rotatedat a speed below the motor speed and in a fixed relationship of say l:l0or thereabout. Liquid may be supplied to the cylinder through anelectromagnetically actuated inlet valve and removed by an electricmotor-driven pump. Liquid in the cylinder may be heated by an electricsheathed wire heating element mounted in a lower wall of the cylinder,the heating element being disposed within the cylinder.

in operation of the washing machine its various functions aresequentially controlled by a program controller comprising a synchronouselectric timer motor actuating, via a stepping linkage, a number of camsand hence switches which control the supply of electrical power to thevarious parts of the machine in sequence so as to cause a particularprogram of operations to be performed on clothes placed in the drum. anda solid state switching circuit, also controlled by the timer motor forthe drive motor of the machine, which controls the speed andacceleration of the drum motor.

With reference now to FIG. 1, which shows such a circuit, the drum ofthe washing machine (not shown) is rotatably driven by a DC electricseries motor 1. The machine is connected to a source of AC supplythrough a main isolating switch (not shown) the motor being suppliedthrough a thyristor 2 and a low value resistor 3. A diode 4 connected inparallel with the motor acts as a flywheel diode in known manner toprovide a smoothing effect on the motor current supply, and a seriescombination of a resistor 38 and capacitor 39 is also connected acrossthe motor to suppress commutator sparking.

The thyristor 2 is fired by a pulse-producing circuit (indicated bychain-dotted lines A) whose output (to the thyristor) is controlled by anegative feedback circuit 8,, sensitive to rising motor speed, apositive feedback circuit 8,, sensitive to rising motor current and atime dependent circuit C, these control circuits acting to vary thepulses produced by circuit A and hence the pulses of DC power suppliedto the motor from the thyristor. The circuits A, B,, B and C aresupplied with low voltage DC by means of a voltage dropping resistance61, a diode 40, a smoothing capacitor 41 and a zener diode 17.

The time dependent circuit C is switched into operation, when required,by means of a switch 5 actuated by a cam in a program controller drivenby a synchronous motor 6. The motor 6 also determines the condition ofother cam actuated switches 7 and 8 in the electrical supply to a heater9 and a pump motor 10.

A further cam actuated switch 29 is arranged to reverse position atregular intervals throughout the washing action in order to periodicallyreverse the drive motor and hence the wash drum in a manner to bedescribed.

In operation the program controller selects, by sequential operation ofthe switches, one of several operations to suit the clothes beingprocessed. A complete washing and drying cycle may, for example,comprise the following steps:

Prewash, spin, wash, spin, rinse and spin.

During the prewash, wash and rinse functions the drum is rotated at lowspeeds (say 50 r.p.m.) in one direction for a short period of time (sayseconds) and then rotated in the opposite direction for an equal periodof time. Since the speed of rotation during these operations is low, thecentrifugal force on the clothes is not sufficient to hold them incontact with the wall of the drum as they are carried round and as aresult they tumble within the drum. The spin action is achieved byrotating the drum at a high speed (say 1,000 r.p.m.) in one directiononly, the centrifugal force on the clothes being sufficiently large toretain them in contact with the wall of the drum and to extract anycontained liquid from them.

The low speed of rotation of the drum is determined by thepulse-producing circuit A and the feedback circuits B, and B The timedependent circuit C is, at this time, switched off by the switch 5,which is closed so as to provide a biassing voltage zero with respect toits emitter to the base of the transistor 11 within the time dependentcircuit, so as to .switch it off, the circuit then being isolated fromthe pulse producing circuit A.

Thus, during a washing (or rinsing) operation, the drive motor speed isdetermined entirely by circuits A, B, and B which are set so as tomaintain the motor speed at substantially 500 rpm, corresponding to adrum speed of 50 rpm, irrespective of normal variations in load on themotor.

The pulse producing circuit A comprises, essentially a capacitor 12, acomplementary pair of transistors 13 and 14, a further transistor 19 anda differentiating circuit comprising resistor 43 and a further capacitor44. The circuit A is supplied with a pulse of approximately rectangularform during each positive half cycle of the AC supply, the negativehalfcycle being suppressed by a zener diode 17. During each saidpositive half-cycle the capacitor 12 becomes charged via diode 45 at arate dependent upon the voltage applied to the base of the transistor 19and the value of the resistor 42, and

governed by the differentiating circuit, the transistor 19 beingconductive at this time, due to the voltage at its emitter being belowthat at its base, this base voltage being normally made up of anadjustable reference voltage derived from a potentiometer network P andmodified by the two feedback circuits B, and B as will subsequently bedescribed.

When the voltage across the capacitor 12 becomes large enough, this willcause the transistor 13 to conduct. The transistor 13 then supplies basecurrent to transistor 14 turning it on so as to supply more base currentto transistor 13. This cumulative action leads to rapid saturation ofthese transistors with the result that the lower end of capacitor 12 iseffectively connected to the H.T. rail via transistors 13 and 14,resulting in a voltage pulse which is now positive with respect to theH.T. rail being applied to the gate of the thyristor 2. Diode 45 nowprevents capacitor 12 from short circuiting directly through transistors13 and 14 and as a result the thyristor switches on, supplying the motorwith a pulse of DC power.

Control of the conduction angle or mark to space ratio for the motorcurrent waveform is effected by varying the rate at which capacitor 12charges which is in turn affected by varying the voltage at the base ofthe transistor 19. As this voltage rises the rate of charge of thecapacitor 12 increases, with the result that the firing pulse to thegate of the thyristor is applied earlier in the half-cycle with theresult that the conduction angle of the supply to the drive motor isincreased giving a faster motor speed.

Similarly if the rate of the charge of the capacitor 12 decreases as aresult of the base voltage of the transistor 19 being reduced the firingpulse to the gate of the thyristor is applied later in the half-cyclecausing the conduction angle of the supply to the drive motor beingdecreased and reducing the motor speed.

As previously pointed out the voltage at the base of the transistor 19is controlled by the feedback circuits B, and B The negative feedbackcircuit B, comprises a tachogenerator 18 (mechanically connected to thedriving motor 1) which applies a voltage, decreasing with increasingmotor speed, to the base of a transistor 19. As the voltage applied tothe base of transistor 19 decreases the power supplied to the motor andhence the motor speed decreases.

The positive feedback circuit B comprises resistors 3, 20 and 21 anddiode 46, the arrangement being such that as the motor load, and hencethe current required for the motor, increases, the positive feedbackvoltage applied to the base of the transistor 19 also increases so thatthe power input to the motor increases to maintain the motor speed.

The feedback circuits B, and B and the value of the reference voltageare chosen so as to maintain the motor speed at approximately 500 r.p.m.at this time, i.e., with circuit C isolated.

At the end of a washing (or rinsing) period the timer motor 6 closes theswitch 16 so as to energize the electric motor driven pump 23 to removeall the free washing liquid from the machine. Simultaneously the timeropens switch 5 which controls the time dependent circuit C. When switch5 is opened a capacitor 24 commences to discharge through a high valueadjustable resistor 25. As a result the voltage at the base of thetransistor 11 increases slowly over a period of time (which can bedetermined by adjustment of the high value resistor 25). The transistor11 is then switched on and thereafter the current through the transistor(from collector to emitter) increases slowly thus increasing the controlvoltage applied at the base of transistor 19 so that the motor speed,and hence the drum speed, slowly increases, as shown in FIG. 2.

During this slow acceleration from washing speed the centrifugal forceon the clothes gradually increases until it exceeds the gravitationalforce on the clothes. This gradual increase in speed has the effect ofslowly moving the clothes outward to contact the wall of the drum in abalanced manner.

As a transistor 11 becomes more conductive the voltage at the base of afurther transistor 27 rises this causing the transistor 27 to startconducting and the voltage at the base of the transistor 19 to riserapidly to a maximum determined by the transistor 27 reaching a stableconducting condition.

As this base current further increases the transistor 19 turns on hardermore rapidly, the conduction angle of the rectified supply to the motorsimilarly increasing rapidly to its maximum, and the motor is suppliedwith maximum power and rapidly accelerates the clothes drum to highspeed thereby spinning the clothes as shown by F in FIG. 2; the point ofconduction of the transistor 27 is represented in FIG. 2 by T. Thecapacitor and resistor values can readily be selected, in dependenceupon the transistors employed, to obtain the appropriate time intervals.A capacitor 60 primarily prevents the voltage at the base ofthetransistor 19 from changing too rapidly and giving an excessivelyfast change in motor speed and also serves to absorb any transientsurges. For terminating the spinning operation (or washing operation)the timer closes a switch S thus reducing the voltage at the base of thetransistor 19 to a value such that it is switches off, gate pulses nolonger being applied to the thyristor 2.

In some cases for example when spin drying delicate materials a shorterspinning time may be required. This is achieved by closure of switch SS,causing a capacitor 31 to charge up via a resistor 50 and diode 51 andthis takes place before transistor 27 starts to conduct, i.e., duringthe slow acceleration period-see FIG. 2. When transistor 27 breaks overthe voltage at its collector decreases and diode 51 is now reversebiassed thus preventing further charge to capacitor 31.

Capacitor 31 now discharges via variable resistor 32 over a period oftime (e.g., l5 seconds) preset by the variable resistor 32.

As capacitor 31 discharges the voltage at the junction of diode 51capacitor 31 and variable resistor 32 decreases and a point is reachedwhen transistor 33 turns off. The voltage at the collector of transistor33 increases and since this is connected to the base of the transistor13 in the pulse producing circuit A the latter will be prevented fromfiring and hence will prevent the application of a firing pulse to thethyristor 2, thus terminating the spinning operation.

Conveniently a further normally open switch 30 may be providedassociated with a part of the suspension of the liquidcontainingcylinder. The switch 30 is so positioned that in the event of the drumbecoming out of balance during the spinning operation (or in the eventof the drum being loaded with a single heavy article, such as a blanket,which may require a considerable time to distribute itself correctlywithin the drum) the excessive movement of the drum on high-speedrotation momentarily closes the switch.

The out-of-balance switch 30 is arranged to turn on a transistor 34having its collector connected to the base of the transistor 11,permitting the capacitor 24 to charge up and reducing the base voltageof the transistor 11 which cuts off, also cutting off the transistor 27and thereby reduces the voltage at the base of the transistor 19. Thislowers the motor speed to a value appreciably below the critical speed.The capacitor 53 will start to charge (having been discharged previouslyby 30) when the switch 30 is opened and the transistor 34 will cut off,recommencing the controlled acceleration from slow speed, to effect aredistribution of the clothes up towards spinning speed in the manneralready described, this operation being repeated if this is necessaryuntil the clothes are distributed in a manner which avoids excessive outof balance. Capacitor 52 and associated circuitry also provides a softstart facility when the motor is initially energized.

It will also be appreciated that if for any reason the motor is switchedoff in any running condition by interrupting the main power supply thecapacitors 24, 26, will discharge completely. On restoring the supplywith the switch 5 open the capacitor 24 will ensure that a graduallyincreasing voltage will be applied to the base of the transistor 11causing the emitter voltage to increase slowly and the controlledacceleration will thus recommence from the slow-speed condition as inthe manner above described, thus avoiding restarting of the motor atfull power which would inflict a severe shock load on the machinetransmission. Diode 53 provides the discharge path for capacitor 52 inthe event of the supply being interrupted as described. 7

The control arrangement conveniently includes safety means for switchingoff the motor 1 in the event of the motor temperature rising to anexcessively high value. Such a safety means may, for example,incorporate (not shown) a negative temperature coefficient of resistanceresistor (an N.T.C.R. resistor) associated with the motor statorwindings such that if the motor temperature rises unduly the currentsupply to the motor 1 is reduced, an excessively high rise cutting offthe motor current completely. A further N.T.C.R. resistor 35 connectedto the emitter of transistor 19 is included as a means of providingtemperature stability for the electronics circuit in known manner. A

Similarly a further safety means is provided to limit any excessive risein motor current. This incorporates a further transistor 36 which isnormally conducting but which in the event of an excessive rise in motorcurrent, resulting in a corresponding rise in the voltage across theresistor 3, cuts off thereby lowering the voltage at the base of thetransistor 19, with the result that the thyristor firing angle ismodified thereby limiting the motor current to a safe value. The valueat motor current reached before the transistor 36 switches off is set byan adjustable resistor 37.

Although the invention has been described in relation to a combinedwashing and spin-drying machine it can also be applied to a machinedesigned solely for the spin drying of clothes in which the clothes drumis arranged to start at a very low speed and then accelerated to a muchhigher spin drying speed.

I claim:

1. A clothes-washing and spin-drying machine or a spin-drying machineincluding:

a. a clothes drum arranged torotate about an nonvertical axis;

b. a DC electric motor for rotating the clothes drum at a relativelyslow speed and also at a substantially higher speed for spin drying;

c. a controlled semiconductor device, connected in series with theelectric motor with respect to input terminals which are arranged to beconnected to a source of alternating current for controlling said motor;and

d. a motor speed control circuit for controlling the operation of thesemiconductor device such that, for carrying out a spin-dryingoperation, the motor is caused to accelerate relatively slowly through acritical speed at which the centrifugal force on the clothes within thedrum equals the force due to gravity and is subsequently caused toaccelerate more rapidly until said higher speed is attained.

2. A machine according to claim 1, wherein the motor speed controlcircuit includes means for generating a series of electric pulses, theduration of which governs the power supply to the motor, and means forvarying the duration of said pulses such as to produce said differentaccelerations.

3. A machine according to claim 2, wherein the electric pulses arearranged to be applied to a control electrode of the semiconductordevice in appropriate alternate half-cycles of the alternating currentsupply so as to fire the device, whereby to cause driving power to besupplied to the motor.

4. A machine according to claim 3, wherein the motor speed controlcircuit includes feedback means which normally tends to maintain themotor rotating with a selected constant slow speed, and time-dependentmeans for overriding the feedback means when a spin-drying operation isinitiated.

5. A machine according to claim 4, wherein the feedback meansincorporates a first feedback circuit deriving from a tachometer coupledto the motor a first feedback signal varying with increasing motor speedand applied to said motor speed control circuit in such a sense as toreduce the motor speed, and a second feedback circuit deriving from themotor current a second feedback signal varying with increasing motorspeed and applied to the motor speed control circuit in such a sense asto increase the motor speed, the feedback signals being so related as tostabilize the motor at said selected slow speed.

6. A machine according to claim including means for supplying areference voltage to the motor speed control circuit to cause it togenerate said series of pulses for driving the motor, the feedbacksignals being arranged to modify said reference voltage in theappropriate sense.

7. A machine according to claim 6, wherein the motor speed controlcircuit includes a capacitor, means for charging the capacitor duringappropriate alternate half cycles of the supply voltage at a ratedependent upon the feedback signals, and means responsive to the chargeon the capacitor for generating a said electric pulse when the charge onthe capacitor reaches a predetermined value.

8. A machine according to claim 7 including a pair of transistorsnormally nonconducting one of which transistors is connected so as tobecome conducting when the charge on the capacitor reaches saidpredetermined value, and being so interconnected with the other byregenerative feedback connections that rapid saturation of bothtransistors takes place resulting in the generation of a said electricpulse.

9. A machine according to claim 6, wherein the time-dependent means isarranged to modify the reference voltage applied to the motor speedcontrol circuit in such a sense as to produce a relative slowacceleration of the motor through said critical speed, and subsequentlya relative rapid acceleration of the motor up to said higher speed.

10. A machine according to claim 9 wherein the time-dependent circuitincludes a first transistor, capacitor means controlling theconductivity of the transistor so as to produce a relatively slowvariation thereof following the actuation of the time dependent circuit,and means responsive to the conductivity of the transistor for modifyingthe reference voltage in such a sense as to produce said relative slowacceleration of the motor, and a second transistor connected so as to beswitched on after the conductivity of the first transistor has reached apredetermined value and to rapidly attain a condition of maximumconduction, and means responsive to the conduction of said secondtransistor for further modifying the reference voltage in such a senseas to produce said relative rapid acceleration of the motor.

11. A machine according to claim 1 including means responsive to anout-of-balance condition of the drum following the acceleration of themotor past the critical speed, and the motor speed control circuit beingresponsive to the operation of the out-of-balance means in the event ofan excessive out-of-balance condition to reduce the motor speed to avalue significantly below the critical speed, and to repeat therelatively slow and relatively rapid acceleration of the motor towardssaid higher speed.

1. A clothes-washing and spin-drying machine or a spin-drying machineincluding: a. a clothes drum arranged to rotate about a nonverticalaxis; b. a DC electric motor for rotating the clothes drum at arelatively slow speed and also at a substantially higher speed for spindrying; c. a controlled semiconductor device, connected in series withthe electric motor with respect to input terminals which are arranged tobe connected to a source of alternating current, for controlling saidmotor; and d. a motor speed control circuit for controlling theoperation of the semiconductor device such that, for carrying out aspindrying operation, the motor is caused to accelerate relativelyslowly through a critical speed at which the centrifugal force on theclothes within the drum equals the force due to gravity and issubsequently caused to accelerate more rapidly until said higher speedis attained.
 2. A machine according to claim 1, wherein the motor speedcontrol circuit includes means for generating a series of electricpulses, the duration of which governs the power supply to the motor, andmeans for varying the duration of said pulses such as to produce saiddifferent accelerations.
 3. A machine according to claim 2, wherein theelectric pulses are arranged to be applied to a control electrode of thesemiconductor device in appropriate alternate half-cycles of thealternating current supply so as to fire the device, whereby to causedriving power to be supplied to the motor.
 4. A machine according toclaim 3, wherein the motor speed control circuit includes feedback meanswhich normally tends to maintain the motor rotating with a selectedconstant slow speed, and time-dependent means for overriding thefeedback means when a spin-drying operation is initiated.
 5. A machineaccording to claim 4, wherein the feedback means incorporates a firstfeedback circuit deriving from a tachometer coupled to the motor a firstfeedback signal varying with increasing motor speed and applied to saidmotor speed control circuit in such a sense as to reduce the motorspeed, and a second feedback circuit deriving from the motor current asecond feedback signal varying with increasing motor speed and appliedto the motor speed control circuit in such a sense as to increase themotor speed, the feedback signals being so related as to stabilize themotor at said selected slow speed.
 6. A machine according to claim 5including means for supplying a reference voltage to the motor speedcontrol circuit to cause it to generate said series of pulses fordriving the motor, the feedback signals being arranged to modify saidreference voltage in the appropriate sense.
 7. A machine according toclaim 6, wherein the motor speed control circuit includes a capacitor,means for charging the capacitor during appropriate alternate halfcycles of the supply voltage at a rate dependent upon the feedbacksignals, and means responsive to the charge on the capacitor forgenerating a said electric pulse when the charge on the capacitorreaches a predetermined value.
 8. A machine according to claim 7including a pair of transistors normally nonconducting one of whichtransistors is connected so as to become conducting when the charge onthe capacitor reaches said predetermined value, and being sointerconnected with the other by regenerative feedback connections thatrapid saturation of both transistors takes place resulting in thegeneration of a said electric pulse.
 9. A machine according to claim 6,wherein the time-dependent means is arranged to modify the refereNcevoltage applied to the motor speed control circuit in such a sense as toproduce a relative slow acceleration of the motor through said criticalspeed, and subsequently a relative rapid acceleration of the motor up tosaid higher speed.
 10. A machine according to claim 9 wherein thetime-dependent circuit includes a first transistor, capacitor meanscontrolling the conductivity of the transistor so as to produce arelatively slow variation thereof following the actuation of the timedependent circuit, and means responsive to the conductivity of thetransistor for modifying the reference voltage in such a sense as toproduce said relative slow acceleration of the motor, and a secondtransistor connected so as to be switched on after the conductivity ofthe first transistor has reached a predetermined value and to rapidlyattain a condition of maximum conduction, and means responsive to theconduction of said second transistor for further modifying the referencevoltage in such a sense as to produce said relative rapid accelerationof the motor.
 11. A machine according to claim 1 including meansresponsive to an out-of-balance condition of the drum following theacceleration of the motor past the critical speed, and the motor speedcontrol circuit being responsive to the operation of the out-of-balancemeans in the event of an excessive out-of-balance condition to reducethe motor speed to a value significantly below the critical speed, andto repeat the relatively slow and relatively rapid acceleration of themotor towards said higher speed.